The invention relates to a method of and an apparatus for measuring inhomogeneities on the wall of boreholes in formations pierced by a borehole, especially for carrying out investigations in fractured and vuggy hydrocarbonbearing and waterbearing rocky reservoirs particularly containing thin rock laminations and, besides, in thinly laminated oil/gas and water reservoirs, and, in thinly laminated stone coal, brown coal, lignite, ore layers by investigating and determining the inhomogeneities on the wall of a borehole.
In the hydrocarbon- and waterbearing rocks the reservoir capacity and permeability are substantially increased by fractures (fissures) and vugs especially in carbonates, in metamorphic and igneous rocks; on the other hand the thin interbedded laminae may obscure the detection of useful minerals by logging methods both in fluid-reservoirs and in coal or ore layers. Hence, the detection of those two objects is of paramount importance in exploring the useful minerals by well logging.
For elucidating the inhomogeneities of the rocks core-drilling is applied, however, the fractures and vugs are partly lost during core-drilling since the cores are broken just at these places; on the other hand delineation of the thin laminae can not be well defined by laboratory investigation and measurement of the cores of small sizes, i.e. the same disadvantages should be taken into account in comparison with the in situ investigations in the boreholes as in other methods of determination of rock physical parameters.
Detection of rock inhomogeneities on the wall of the borehole has not yet been adequately solved in the well logging methods. This was the subject of a Hungarian patent issued in 1966 under the Patent No. 152 234, and of the U.S. Pat. No. 4,567,759 granted in 1986. All these referenced patents propose the measurement of the variations of the electrical conductivity of the rocks by pad-type microsondes and they exhibit a common drawback resulting a serious shortage in the task solution, namely they can not distinguish the effects of the fractures and vugs and thin rock laminae from other kinds of effects which also influences the electrical conductivities of the rocks, thus e.g. from variations of porosity and the dispersed clay content of the rock matrix.